An electromagnetic fuel injector comprises a tubular housing member inside which there is defined an injection chamber delimited at one end by an injection nozzle which is controlled by an injection valve governed by an electromagnetic actuator. The injection valve is provided with a shutter, which is rigidly connected to a movable anchor of the electromagnetic actuator so as to be moved under the action of said electromagnetic actuator between a closed position and an open position of the injection nozzle against the action of a closing spring that tends to hold the shutter in the closed position.
The injection valve is normally closed due to the effect of the closing spring which pushes the shutter into the closed position, in which the shutter presses against a valve seat of the injection valve and the anchor is spaced apart from a fixed magnetic armature of the electromagnetic actuator. To open the injection valve, that is to move the shutter from the closed position to the open position, a coil of the electromagnetic actuator is energized so as to generate a magnetic field which attracts the anchor towards the fixed magnetic armature against the elastic force exerted by the closing spring; in the opening phase, the stroke of the anchor ends when said anchor impacts against the fixed magnetic armature. In other words, in the opening phase of the injection valve the anchor accumulates kinetic energy which is subsequently dissipated in the impact of the anchor against the fixed magnetic armature.
When the fuel is liquid (for example petrol or diesel) the kinetic energy of the anchor is partly dissipated by the action of the fuel present between the anchor and the fixed magnetic armature; in other words, the movement of the anchor is slowed down by the fuel present between the anchor and the fixed magnetic armature which must be moved by the movement of the anchor to allow said anchor to come into contact with the magnetic armature. Consequently, when the fuel is liquid the impact of the anchor against the fixed magnetic armature is not excessively violent and does not therefore cause any appreciable wear on said components.
On the other hand, when the fuel is gaseous, (for example methane or mixtures of propane and butane), the braking action of the fuel on the anchor described above is almost non-existent and the impact of the anchor against the fixed magnetic armature is therefore particularly violent. Consequently, in fuel injectors for gaseous fuels the reciprocal contacting surfaces of the anchor and of the fixed magnetic armature are frequently subject to a considerable amount of wear with a subsequent loss of material which results in the lengthening of the anchor stroke and alters the functional characteristics of the injector. Said wear is thus eventually the cause of significant variations in the functional characteristics of the injector, making proper injection control difficult, if not impossible, both in terms of the instant in which injection starts and in terms of the amount of fuel that is injected.
A solution that has been proposed to overcome the drawbacks described above consists of interposing an element made of resilient material (e.g. elastic) between the anchor and the fixed magnetic armature. Said element can be fitted, without distinction, to the anchor or to the fixed magnetic armature, in order to limit the mechanical stress on these components when the anchor impacts against the fixed magnetic armature. However, it has been observed that the element made of resilient material tends to wear out very quickly due to the effect of the anchor continuously impacting against the fixed magnetic armature, limiting the efficiency of this structural solution.
One possible solution to this problem is to increase the thickness of the element made of resilient material in order to give said element made of resilient material greater mechanical strength and better wear resistance. However, increasing the thickness of the component made of resilient material inevitably also increases the size of the magnetic gap between the anchor and the fixed magnetic armature (the resilient material is inevitably non-ferromagnetic) and thus makes it necessary to increase the number of ampere turns of the electromagnetic actuator with a subsequent increase in the cost, weight, overall dimensions and electric power consumption of the electromagnetic actuator.
Patent applications DE102004037250A1 and US2005017097A1 describe an electromagnetic fuel injector comprising an injection nozzle controlled by an injection valve; a movable shutter to control the flow of fuel through the injection valve; an electromagnetic actuator, which is suitable to move the shutter between a closed position and an open position of the injection valve and comprises a fixed magnetic pole, a coil suitable to induce a magnetic flux in the magnetic pole, and a movable anchor suitable to be magnetically attracted by the magnetic pole; an absorption element, which is made of amagnetic elastic material; and a protective element, which is coupled to the absorption element and has the function of protecting the absorption element against the action of the fuel flowing under pressure against the absorption element through delivery holes in the anchor.
The effective functional characteristics of an electromagnetic fuel injector must not differ from its nominal functional characteristics (i.e. expected and desired characteristics) by more than a fixed percentage (generally by not more than a small percentage) defined in the project design stage. To comply with this requirement and compensate for the inevitable constructional tolerances of all the components, at the end of the production line the electromagnetic fuel injectors are adjusted or calibrated during an operation which normally consists of adjusting the pre-load of the closing spring (i.e. the elastic force generated by the closing spring). In particular, in electromagnetic fuel injectors the pre-load of the closing spring is adjusted so that the effective injection rate is equal to the nominal injection rate.
However, it has been observed that by adjusting the pre-load of the closing spring it is possible to obtain an effective injection rate that is equal to the nominal injection rate, although this produces a significant fluctuation in the dynamic characteristics of the fuel injectors. In other words, although the high fluctuation of the pre-load of the closing spring obtained by performing the calibration described above makes it possible to standardize the effective injection rate (i.e. fuel injector behaviour in the stationary condition), it also causes notable differences in the dynamic characteristics of the fuel injectors (i.e. fuel injector behaviour in the transient state). Said differences in the dynamic characteristics make it very complicated to control a fuel injector to perform very short injections (for instance as in the sequence of pilot injections preceding the main injection) in which said fuel injector is always in the transient state.